Epoxy resins are used mainly in many applications relating to paints, civil engineering works, and electrical works for their excellent electrical properties, adhesive properties, heat resistance, and the like. In particular, aromatic epoxy resins such as bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, phenol novolak epoxy resins and cresol novolak epoxy resins show excellent water resistance, adhesive properties, mechanical properties, heat resistance, electrical insulating properties, and economics and are used widely in combination with a variety of curing agents. However, these resins degrade easily by the action of ultraviolet rays and the like as they contain aromatic rings and have been restricted in applications requiring light resistance.
In the rapid-growing area of LED devices, aromatic epoxy resin compositions are used at times as an encapsulating material in the fabrication of devices not involving ultraviolet rays nor blue light in consideration of heat resistance, adhesive properties, and light resistance. However, developmental works are under way to raise the output of LED devices by the use of ultraviolet rays or blue light and, if an aromatic epoxy resin composition were used as an encapsulating material, such an encapsulant would degrade because of a deficiency in resistance to ultraviolet rays or blue light and turn yellow with the lapse of time to lower the brightness of LED devices.
The prior-art documents relating to this invention are listed below.
Patent document 1: JP3537119 B
Patent document 2: JP3415047 B
Patent document 3: JP2003-27774732 A
Epoxy resin compositions for use as electrical and electronic materials comprising hydrogenated epoxy resins obtained by hydrogenating aromatic epoxy resins and curing agents are proposed in the patent document 1. Curable epoxy resin compositions comprising hydrogenated epoxy resins obtained by hydrogenating aromatic epoxy resins, cyclic aliphatic epoxy resins, and curing agents are proposed in the patent document 2. Epoxy resin compositions for encapsulating LEDs are proposed in the patent document 3; the compositions comprise hydrogenated epoxy resins having an epoxy equivalent of 230-1000 g/eq obtained by hydrogenating aromatic epoxy resins or epoxy resins having an epoxy equivalent of 230-1000 g/eq obtained by reacting hydrogenated epoxy resins obtained by hydrogenating aromatic epoxy resins with polycarboxylic acids, alicyclic epoxy resins obtained by epoxidizing cyclic olefins, and acid anhydride-based curing agents or cationic polymerization initiators. Although the proposed resin compositions produce a certain improvement in light resistance and heat resistance, there is a demand for epoxy resin compositions that shrink less on curing and show more improved light resistance. Furthermore, depending upon the method of molding, there is another demand for resin compositions of good curability, light resistance, and heat resistance that are solid at ordinary room temperature and become fluid on heating.